tcg-s390: Improve setcond

There are a variety of common cases for which we can use carry tricks to
avoid a conditional branch.  On very new hardware, use LOAD ON CONDITION
instead of a conditional branch.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-s390: Don't force -march=z990

While we still require the LONG DISPLACEMENT facility, defaults
have moved on since then.  Don't override the system compiler,
whose default may be set to z9-109 or later.

Signed-off-by: Richard Henderson <rth@twiddle.net>